Typically, an automotive air control system comprises a multispeed blower motor, a blower operable by the motor for delivering air to the cab of the vehicle, heater means for selectively and adjustably heating the air, air conditioning means for selectively cooling and dehumidifying the air, and a recirculation door or other recirculation control device for selecting between outside air and recirculation of the air in the cab of the vehicle. The air conditioning is an option which may be omitted in some instances.
The typical air control system comprises a control apparatus which may employ a multiposition rotary blower switch for controlling the energization and speed of the blower motor. A heater control device is generally provided for shutting off the heat or supplying a variable amount of heat to the air delivered to the cab of the vehicle. Typically, the air conditioning is controlled by an air conditioning (A/C) switch which selectively energizes and de-energizes an air conditioning clutch whereby an air conditioning compressor is selectively driven by the propulsion motor of the vehicle.
Typically, the recirculation door or other control device is operated by a vacuum motor or operator which is adapted to be energized by vacuum from the air intake of the propulsion motor. When the vacuum motor is de-energized, the recirculation door is generally positioned to deliver outside air to the air control system. When the vacuum motor is energized, the recirculation door is moved to a position in which the intake of outside air is completely or partially shut off, and the air in the cab of the vehicle is recirculated through the air control system. The supply of vacuum to the vacuum motor may be controlled by a separate manually operable vacuum switch or valve which typically also controls other vacuum motors for directing the heated or cooled air to floor vents, panel vents and windshield defrosting vents. In some instances, a manually operable electrical switch may be employed to control the vacuum motor which switches between outside air and recirculation. In that case, a relay valve is provided having a solenoid for actuating a vacuum valve. When the electrical switch is closed, the solenoid is energized whereby the vacuum valve is operated to supply vacuum to the vacuum motor.